In modern day automotive design, parts are being machined to ever closer and more consistent tolerances to increase engine efficiency and reliability. In the machining of automotive internal combustion engine blocks and cylinder heads, a boring or reaming operation must be performed to produce two or more camshaft journal bores which act as load bearing surfaces for a rotating camshaft. Frequently, two horizontal boring machines operate in tandem to bore the journals for both the intake and exhaust camshafts nearly simultaneously.
A prior commonly known camshaft journal boring machine has a drive spindle and a slide for rotating and longitudinally reciprocating a generally cylindrical boring bar. The boring bar has a plurality of cutting inserts longitudinally spaced apart the distance between camshaft journals for simultaneously machining all journals concentric with the axis of rotation of the boring bar.
To bore a series of camshaft journals in a cylinder head, the cylinder head is transferred into a workstation of this boring machine. The platen of the boring machine is rapidly advanced toward the workpiece to advance the boring bar through the journal holes until each cutting insert is completely beyond its associated journal to be bored, and the distal end of the bar is received for rotation in a support bushing which radially constrains the end of the boring bar. Depending upon bar length and the required dimensional accuracy of the journal bores, a second independent support bushing may engage the boring bar near its middle to minimize boring bar flexure and tool chatter. The workpiece is then lowered onto locators and clamped in the workstation. This aligns the axis of rotation of the boring bar with the axes of the camshaft journals to be machined. If necessary, the individual cutting inserts are adjusted to the proper boring diameter. To simultaneously bore all the camshaft journals, the boring bar is rotated by the spindle while being out-fed by retraction of the slide. After the boring operation is completed, bar rotation is stopped, and the bar is latched or locked in a predetermined position to prevent bar rotation and to assure clearance between the inserts and the workpiece, when raised, to prevent the inserts from scarring or re-cutting the finished bores when the bar is retracted from the workpiece. To complete the machining cycle, the workpiece is unclamped and raised, the bar is released from all bushing supports and rapidly retracted to its starting position, the machined cylinder head is then transferred from the workstation and another cylinder head is transferred into the workstation to be machined.
This prior art machine for boring camshaft journals suffers from boring bar flexure, vibration and cutting tool chatter which results in machined journals having taper, concentricity, and roundness errors as well as poorer surface finish. Improper diametral adjustment of any of the cutting inserts can result in a series of camshaft journals bored to different diameters and tolerances causing premature camshaft or bearing failure, excessive vibration, or lubrication problems. Dynamic imbalance, producing boring bar vibration and chatter, causing concentricity, and out-of-roundness errors, is also common because the cutting inserts are axially aligned on the bar creating a non-uniform distribution of mass and cutting forces. An increased likelihood of chatter and excessive bar flexure also results from the long bar being inadequately radially constrained and having a substantially smaller diameter than the journal bore diameter.
The problems commonly encountered in this boring apparatus result in the costly scrapping of numerous cylinder heads or blocks. Even worse, defectively machined journals not detected by the inspection process will receive camshafts and be installed in internal combustion engines which in use may suffer excessive camshaft vibration, premature bearing failure, or poor or erratic engine timing which reduces engine efficiency, durability and reliability and increases engine warranty and repair costs.